PIA11666: Shadow Reaches the A Ring

A vertically extended structure or object in Saturn’s F ring casts a
shadow long enough to reach the A ring in this Cassini image taken just
days before planet’s August 2009 equinox.

The structure can be seen as a bulge within the bright core of the F ring
on the left of the image. The structure rises far enough above the ring
plane for the shadow to be cast across the Roche Division and onto the A
ring. The shadow is barely visible stretching across the top right
quadrant of the image. The shadow appears very faint here because this
view looks toward the unlit side of the rings.

This image and others like it (see PIA11662 and PIA11663) are only possible
around the time of Saturn’s equinox which occurs every half-Saturn-year
(equivalent to about 15 Earth years). The illumination geometry that
accompanies equinox lowers the sun’s angle to the ring plane and causes
out-of-plane structures to cast long shadows across the rings. Exact
equinox at Saturn begins August 11, 2009, and lasts about four days.
Shadows have grown longer as those days draw near. Cassini’s cameras have
spotted not only the predictable shadows of some of Saturn’s moons (see
PIA11657), but also the shadows of newly revealed vertical structures in
the rings themselves (see PIA11654).

The A ring in the first (top) image has been brightened relative to F ring
to enhance visibility of the ring and shadow. The entire image in the
second (bottom) version has been contrast enhanced. Bright specks in the
image are background stars.

This view looks toward the unilluminated side of the rings from about 28
degrees above the ring plane.

The image was taken in visible light with the Cassini spacecraft
narrow-angle camera on July 30, 2009. The view was obtained at a distance
of approximately 1.8 million kilometers (1.1 million miles) from Saturn
and at a Sun-Saturn-spacecraft, or phase, angle of 96 degrees. Image scale
is 10 kilometers (6 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European
Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory,
a division of the California Institute of Technology in Pasadena, manages
the mission for NASA’s Science Mission Directorate, Washington, D.C. The
Cassini orbiter and its two onboard cameras were designed, developed and
assembled at JPL. The imaging operations center is based at the Space
Science Institute in Boulder, Colo.